Equity Investments (Details 2) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2012	Dec. 31, 2011	Mar. 31, 2011	Dec. 31, 2010
Balance Sheets:
Real estate properties, net	$ 6,515,601	$ 6,310,062
Acquired real estate leases, net	374,022	343,917
Cash and cash equivalents	193,587	192,763	25,389	194,040
Rents receivable, net	226,565	217,592
Other assets, net	210,166	197,346
Total assets	7,713,391	7,447,026
Unsecured revolving credit facility		100,000
Mortgage notes payable	784,454	632,301
Assumed real estate lease obligations, net	72,392	70,179
Other liabilities	140,172	158,272
Shareholders' equity	3,517,424	3,568,517
Total liabilities and shareholders' equity	7,713,391	7,447,026
GOV
Balance Sheets:
Real estate properties, net	1,191,091	1,198,050
Acquired real estate leases, net	112,178	117,596
Cash and cash equivalents	9,275	3,272
Rents receivable, net	26,472	29,000
Other assets, net	24,231	20,657
Total assets	1,363,247	1,368,575
Unsecured revolving credit facility		345,500
Unsecured term loan	350,000
Mortgage notes payable	94,826	95,383
Assumed real estate lease obligations, net	10,482	11,262
Other liabilities	22,034	24,762
Shareholders' equity	885,905	891,668
Total liabilities and shareholders' equity	$ 1,363,247	$ 1,368,575